Step-by-step to a southerly setting sun

8 PM January 1, 2007

The last paragraph of my previous post wasn’t particularly clear. Here’s a step-by-step guide to modelling the Sun’s direction at sunset, using only a ball, a lamp, and a texta.

  1. Put a lamp up one end of the room. This will be the Sun.
  2. Get a ball to represent the Earth. Anything between a tennis ball and a basketball will do.
  3. On the Earth, select a spin axis (I used the inflation nipple as the North pole, but you can draw in the poles if you have to.)
  4. Orient the ball so the spin axis is vertical. The top half of the Earth the northern hemisphere, and the bottom bit the southern hemisphere. Draw in the equator.
  5. Tilt the spin axis of the Earth 23 degrees away from the sun to represent the southern hemisphere summer. The southern pole will now be a little closer to the Sun than the northern pole.
  6. Draw in a line of longitude (i.e a line running from the North pole to the South pole), on the side of the ball closest to the light. On this line, it is noon. The point on the line closest to the light is on the Tropic of Capricorn. (digression: You may notice that the north pole is in shadow, and has no light at midday. If you draw in the line of longitude for midnight, you will find the parts of the southern hemisphere that have midnight sun.)
  7. Pick a point about half way down the Southern Hemisphere and draw in the line of latitude for that point.
  8. Around the side of the ball, where that line of latitude crosses from light into shadow, is a point where sunset (or sunrise) is taking place. If you stand behind the ball and sight back to the light across that point, you will see that, from the Earth’s inhabitant’s point of view, the Sun is south of the line of latitude.

If that doesn’t make sense, please drop me a line. I will come and explain it to you, complete with ball, texta, and lamp for the low, low price of one soft drink.

By alang | # | Comments (1)
(Posted to Python, javablogs and Tall Tales)


At 01:39, 02 Jan 2007 Richard wrote:

If you're going to get precise with your tilt, you should go with 23.5 degrees (really, 23 degrees, 27 minutes, or 23.45 degrees, but who's counting).

However, for a good approximation of the earth's tilt and Sydney's latitude, you can use a 1/4 of 90 degrees (22.5 degrees) and 3/8 of 90 degrees (33.75 degrees) respectively. Halfway as Alan suggest works too, since it exaggerates the effect, and should make it more obvious when using smaller spheres.


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